"If you are OK with using MIT or BSD licensed software you should be OK with using Facebook's BSD + Patents software." (From GP)
I.e. patent grant nullifies, and you are left with a BSD only license.
The main BSD license file says plainly that Facebook grants you the right to use the software. Nowhere does it say that it is merely a grant of copyrights. While the Facebook Additional Grant of Patent Rights purports to be purely additive, its mere existence changes how I would interpret the scope of that main license file. As such, I find the meaning and implications of Facebook's contract to be unclear. I would not want to depend on it, personally.
The feeling is a bit like stepping into a restaurant that offers lead-free burgers for only $0.99 extra. Theoretically, declining the upcharge is no different than eating anywhere else. Yet...
I am not a lawyer. This is not legal advice.
Because you can't just rewrite it in another framework if such a totally contrived case arrive?
Say you are a company A, and have product P that uses react.
You find out that company B infringed on a patent in your other product Q.
You sue B.
It turns out B is owned by FB.
Your product P is in trouble.